Three different missense suppressor mutations isolated in strains of E. coli have been found to affect the GGG-specific glycine tRNA. These mutations alter the codon recognition of the tRNA to AGA, GAG and GAA, respectively. In addition to codon recognition, biosynthesis and other functional aspects of the tRNA, such as aminoacylation and ribosome binding are affected by these mutations. The complete sequence of the wild type tRNA species and one of the mutant species have recently been completed, and current experiments are aimed at determining the change in primary sequence resultant from each of the other mutations, as well as at a complete functional characterization of these mutant tRNA's. This approach should provide insight into structure-function relationships in tRNA. We are also studying E. coli mutants which carry a small tandem duplication in the glyT ion thi purD region. Objectives are the isolation of the small circular DNA which results from a recombinational event between the tandem segments, the characterization of this product and the genetic and physical factors which influence the genetic recombination. In addition we are studying the factors which influence the generation of tandem duplications.